ST is used to copy
the fullword stored in the register specified by operand 1 into the fullword
memory location specified by operand 2.
Consider the following example,
ST R9,AFIELD
In this case, the contents
of register 9 are copied to the fullword in memory denoted by AFIELD. This operation destroys the previous
contents of AFIELD but leaves R9 unchanged.
Since ST is an RX
instruction, an index register may be coded as part of operand 2. Notice that in the previous example, no
index register was specified. When the
index register is omitted, the assembler chooses R0, which does not contribute
to the address. The following example illustrates this idea.
ST R9,AFIELD(R5)
The assembler converts AFIELD to a base register and
displacement, while R5 is the index
register. For instance, the expression
AFIELD(R5) might (we can not determine the base register) be equivalent to the
explicit address 0( R5,R3 ) -
displacement = 0, index register = R5, base register = R3 (see Explicit Addressing). The effective address is computed by adding
the base register contents to the index register contents plus the displacement. If the index register contains an “8”, then
AFIELD(R5) refers to the fullword that begins at an 8 byte displacement from
the beginning byte of AFIELD. The
following examples illustrate several explicit addresses that include an index
register.
In the first explicit address, 4( R4, R3), the effective address
is computed by adding the contents of base register 5, the contents of index
register 3, and the displacement ( 1000 + 4 + 4 = 1008 ). The second address 0( R5, R3) is computed as
1000 + 8 + 0 = 1008, and the third address, 4( R5, R3 ) is computed to be 1000
+ 8 + 4 = 100C (hexadecimal).
If an index register is not explicitly coded, as in the
instruction “ST R9,AFIELD”, the
assembler chooses R0 as the index register, which does not contribute to the
effective address.
Some Unrelated ST’s
R7 = X’00001000’
R8 = X’00000004’
R9 = X’00000008’
AFIELD
DC F’20’ AFIELD = X’00000016’
BFIELD
DC F’-1’ BFIELD = X’FFFFFFFF’
CFIELD
DC F’0’ CFIELD = X’00000000’
ST R7,AFIELD AFIELD =
X’00001000’
ST R8,AFIELD AFIELD =
X’00000004’
ST R8,BFIELD BFIELD =
X’00000004’
ST R7,AFIELD(R8) CHANGES
BFIELD TO X’00001000’
ST R7,AFIELD(R9) CHANGES
CFIELD TO X’00001000’
1) Operand 2 should denote a fullword in memory. It is possible to store the contents of a
register into 4 bytes of memory that are not aligned on a fullword, but the
assembler will warn you that operand 2 is not properly aligned. If the field involved cannot be aligned
conveniently, consider using STCM to
copy the contents of a register into memory.